The primary function of a bushing is to carry current through a grounded barrier, such as a wall or an enclosure of an electrical apparatus. The bushing keeps current from passing into the grounded barrier by virtue of its insulating properties. A bushing is built with or without a condenser.
A non-condenser bushing comprises a current carrying center conductor surrounded by a solid, liquid or gas dielectric medium and a ceramic- or elastomeric insulator.
A condenser bushing for medium- and high-voltage has an additional component called an insulating core that aids electrical field distribution along the length of the bushing. The insulating core is built up around a central tube that is in the current carrying path of the bushing. For some types of bushings the central tube is not in the current carrying path of the bushings. The medium- and high-voltage bushing insulating cores are for example constructed of either oil impregnated paper (OIP) or resin impregnated paper (RIP). Wound with the paper is a plurality of equalization plates arranged concentrically within the core. These layers are constructed of metallic foil, preferably aluminum foil, or conductive ink, which serve to control the electrical field internal and external to the bushing assembly.
The resin impregnated paper insulating core may be produced by winding paper and equalization plates on the center tube and then impregnating with a resin in a mould. The resin used in a resin impregnated paper insulating core is for example epoxy. The mould may also be the actual elastomeric sheath that becomes part of the final product assembly. The mould could also be made of paper or metal that is removed after the curing process. When using a removable mould, an elastomeric sheath is extruded directly on to the resin impregnated paper insulating core. The resin impregnated paper insulating core could also be placed inside a hollow glass fiber reinforced epoxy cylinder with an elastomeric sheath extruded directly on its outer surface or placed inside a hollow ceramic cylinder. There are certain constructions that do not require either the elastomeric sheath or the hollow ceramic cylinder after removal from the mould. Outfitting with a mounting flange along with several other components, such as mechanical fittings, possibly an expansion tank, completes the bushing assembly.
The elastomeric sheath made of silicon or EP-rubber, along with the ceramic insulator act to prevent creepage current along the outer surface of the bushing assembly. Both the elastomeric and ceramic insulator have bell shaped protrusions called sheds that increase the creepage distance along its length and further reduce the incidence of creepage current.
When using a hollow glass fiber reinforced epoxy cylinder or a hollow ceramic cylinder as insulator, the space between the insulating core and the outer hollow insulator is filled with a solid, semi-solid, liquid or gaseous dielectric medium. An example of a liquid dielectric medium is oil and an example of a gaseous dielectric medium is SF6.
Epoxy and elastomers absorb moisture when exposed to the atmospheric conditions. Resin impregnated paper bushings with or without elastomeric sheathing extruded directly on its insulating core is susceptible to moisture absorption during long term exposure to atmospheric conditions. Moisture absorption into the insulating core may cause degradation of the dielectric integrity of the bushing and diminish its ability to serve its intended purpose.
To prevent water from reaching the epoxy, there are known temporary solutions employed by the industry such as plastic or desiccants, or a cost prohibitive metal enclosure. There is no cost effective and reliable method known today for having a protective layer that keeps the moisture away from the epoxy. One reason for this is the limited adhesion and temperature stability of such known protective layer.
Therefore there is a need for a bushing where moisture uptake in the condenser core is prevented and a method of manufacturing such a bushing, which is simpler, more economical than known methods, and results in a finished product of high quality.